Practice golf ball

ABSTRACT

A practice golf ball has a core and a cover. The core is made of a rubber composition which includes a base rubber, a co-crosslinking agent which is methacrylic acid, a crosslinking initiator and a metal oxide. The respective ingredients in the rubber composition are formulated in specific amounts, and the core has an optimized hardness profile. The cover includes a resin component which is composed primarily of polyurethane. The cover has a thickness of 0.3 mm to 1.9 mm, and a material hardness, expressed as the Shore D hardness, of 30 to 48. The respective deflections of the core and the ball under a specific load, and the relationship therebetween, are optimized. The volume ratio VR of dimples on the surface of the ball is optimized.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2011-099945 filed in Japan on Apr. 27, 2011,the entire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a practice golf ball which is suitablefor use at such places as driving ranges. More specifically, theinvention relates to a practice golf ball which is endowed with theproperties required of practice balls intended for long-term use,including better durability to cracking and durability of appearancethan ordinary game balls, and the ability to maintain a stable feel onimpact and a stable flight performance over a long period of time.

2. Prior Art

In terms of the performance sought in a practice golf ball, it isgenerally desirable to obtain ball characteristics which are nodifferent from those of golf balls used to play an actual round of golf.If, for example, a practice ball has a feel on impact or a flightperformance which differs from that of a game ball, when the time comesto play an actual round, the golfer will be unable to take fulladvantage of the skills acquired through practice.

On the other hand, because of the size limitations of driving ranges,there is a need today for low-distance practice golf balls in order tokeep the balls from flying out of the driving range. Accordingly, thereexists a desire for practice balls which have a performancecharacterized by the same feel on impact as a game ball, but a shortflight distance.

Moreover, because practice golf balls are used over and over again at adriving range, in order to reduce the costs of operating the range, itis desirable for the balls to be capable of enduring use for as long aperiod of time as possible even when repeatedly hit. That is, becausethe golf balls at a driving range are repeatedly used over a long periodof time by many golfers practicing their skills, there is a desire forsuch practice balls to have a durability, including a durability tocracking, which is superior to that of ordinarily used game balls.

As for the ball structure in a practice golf ball, for the ball to beimparted with a flight performance and feel similar to those experiencedwhen playing an actual round of golf, it is more desirable to use atwo-piece solid golf ball than a one-piece ball.

As is widely known, two-piece solid golf balls are composed of a coreand a cover, with the core being a rubber crosslinked material ofcertain desirable properties obtained by using a base rubber composedprimarily of cis-1,4-polybutadiene rubber to which compoundingingredients such as a co-crosslinking agent, a metal oxide and anorganic peroxide have been added. For example, JP-A 59-49779 teaches, asthe rubber composition for the core of a two-piece solid golf ball, thecompounding of a given amount of zinc methacrylate as a co-crosslinkingagent in cis-1,4-polybutadiene rubber. However, when zinc methacrylateis used in this way in a core-forming rubber composition, achieving goodball durability in long-term use at a golf driving range has beendifficult.

In addition, JP-A 2003-70936 describes, as a rubber composition for thecore of a two-piece solid golf ball, the compounding of a given amountof zinc acrylate in cis-1,4-polybutadiene rubber. However, here too,when zinc acrylate is used in the rubber-forming rubber composition,achieving good ball durability in long-term use at a driving range hasbeen difficult.

Also, JP-A 2004-180793 and JP-A 2008-149190 disclose golf balls whichuse zinc acrylate in the core formulation and use a thermoplasticpolyurethane as the cover material. However, drawbacks of such golfballs include a hard cover, a large decrease in ball flight followingabrasion of the ball surface, and poor durability of markings(“markings” referring here to, for example, a brand name or playernumber printed on the surface of the ball at the time of manufacture).

In view of the foregoing, it is an object of the present invention toprovide a practice golf ball which is endowed with the propertiesrequired of practice balls intended for long-term use, including betterdurability to cracking and durability of appearance than ordinary gameballs, and maintains both a stable feel on impact and a stable flightperformance over a long period of time.

SUMMARY OF THE INVENTION

We have discovered that, in the production of practice golf balls havinga core and a cover, by using methacrylic acid as a co-crosslinking agentin a rubber formulation for the core and, with regard to the hardnessprofile of the core, by specifying and optimizing both the hardnessdifference between the surface and center and the hardness gradient, andmoreover by using a polyurethane resin as a resin material for thecover, owing to the synergistic effects of these features, the resultingballs have better durability to cracking and durability to abrasion thananticipated by golf ball designers. This discovery has made it possibleto obtain a practice golf ball which maintains a good appearance and agood flight performance even with long-term use, and moreover has a goodfeel on impact.

Accordingly, the invention provides a practice golf ball having a coremade of a rubber composition comprising a base rubber and, ascompounding ingredients: a co-crosslinking agent, a crosslinkinginitiator and a metal oxide; and having a cover which encases the coreand includes a resin component. The co-crosslinking agent is methacrylicacid, and the metal oxide is zinc oxide. The core has a hardness profilein which, letting A be the JIS-C hardness at a surface of the core, B bethe JIS-C hardness at a position 2 mm inside the core surface, C be theJIS-C hardness at a position 5 mm inside the core surface, D be theJIS-C hardness at a position 10 mm inside the core surface, E be theJIS-C hardness at a position 15 mm inside the core surface, and F be theJIS-C hardness at a center of the core: A is from 65 to 83, B is from 59to 78, C is from 61 to 80, D is from 59 to 75, E is from 56 to 70, and Fis from 53 to 67. In the above core hardness profile, the relativehardness conditions A>B<C≧D>E>F are satisfied, the value A-F is not morethan 19, the core is formed in such a way that A has the highest valueamong A to F, and the value A-C is from 0 to 8. The core has a specificgravity of from 1.05 to 1.2. The resin component of the cover iscomposed primarily of polyurethane. The cover has a thickness of from0.3 mm to 1.9 mm and a material hardness, expressed as the Shore Dhardness, of from 30 to 48. When the core and the ball are eachcompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf), letting deflection by the core be CH and deflection bythe ball be BH1, the core deflection CH is from 2.8 mm to 7.0 mm and theratio CH/BH1 is from 0.95 to 1.1. The ball has formed on a surfacethereof a plurality of dimples, each dimple having a spatial volumebelow a flat plane circumscribed by an edge of the dimple, and the sumof the dimple spatial volumes, expressed as a percentage (VR) of thevolume of a hypothetical sphere representing the ball were the ball tohave no dimples on the surface thereof, being from 0.95% to 1.7%.

The practice golf ball of the invention preferably has, upon initialmeasurement, a deflection BH1 (mm) when compressed under a final load of1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initialvelocity BV1 (m/s), and, when measured again after being left to standfor 350 days following initial measurement, a deflection BH2 (mm) whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf) and an initial velocity BV2 (m/s), such that thedifference BH2−BH1 is not more than 0.2 mm and the difference BV2−BV1 isnot more than 0.3 m/s.

The practice golf ball of the invention may have formed on a surfacethereof a plurality of dimples which satisfy conditions (1) and (2)below:

(1) the dimples have a peripheral edge provided with a roundnessrepresented by a radius of curvature R of from 0.5 mm to 2.5 mm; and

(2) the ratio ER of a collective number of dimples RA having a radius ofcurvature R to diameter D ratio (R/D) of at least 20%, divided by atotal number of dimples N on the surface of the ball, is from 15% to95%.

The practice golf ball which satisfies above conditions (1) and (2) maysatisfy also condition (3) below:

(3) the ball has thereon a plurality of dimple types of differingdiameter, and the ratio DER of a combined number of dimples DE obtainedby adding together dimples having an own diameter and an own radius ofcurvature larger than or equal to a radius of curvature of dimples oflarger diameter than the own diameter plus dimples of a type having alargest diameter, divided by the total number of dimples N on thesurface of the ball, is at least 80%.

The practice golf ball which satisfies above conditions (1) to (3) maysatisfy also conditions (4) to (6) below:

(4) the number of dimple types of differing diameter is 3 or more;

(5) the total number of dimples N is not more than 380; and

(6) the surface coverage SR of the dimples, which is the sum ofindividual dimple surface areas, each defined by a flat planecircumscribed by an edge of the dimple, expressed as a percentage of thesurface area of a hypothetical sphere representing the ball were theball to have no dimples on the surface thereof, is from 60% to 74%.

The polyurethane in the resin component of the cover may be athermoplastic polyurethane elastomer, in which case the thermoplasticpolyurethane elastomer preferably includes soft segments formed from apolymeric polyether polyol and hard segments formed from an aromaticdiisocyanate.

The practice golf ball of the invention preferably has, upon initialmeasurement, a deflection BH1 when compressed under a final load of1,275 N (130 kgf) from an initial load of 98 N (10 kgf) of from 2.8 mmto 7.0 mm.

The practice golf ball of the invention preferably has, upon initialmeasurement, an initial velocity BV1 of not more than 76 m/s.

In the practice golf ball of the invention, the compounding ingredientsin the rubber composition are preferably included in respective amountsof from 10 to 40 parts by weight of methacrylic acid, from 15 to 30parts by weight of metal oxide, from 0.3 to 0.88 part by weight ofcrosslinking initiator, and from 0.1 to 1.0 part by weight of anantioxidant, per 100 parts by weight of the base rubber.

The practice golf ball of the invention is endowed with the propertiesrequired of practice balls intended for long-term use, including betterdurability to cracking and durability of appearance than ordinary gameballs, and the ability to maintain a stable feel on impact and a stableflight performance over a long period of time.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a schematic cross-sectional diagram of a practice golf ballaccording to one embodiment of the invention.

FIG. 2 is a schematic diagram of a core illustrating positions A to F inthe core hardness profile.

FIG. 3 is a schematic diagram showing an example of a dimplecross-section.

FIG. 4A is a top view and FIG. 4B is a side view showing an example of adimple configuration.

FIG. 5 is a top view showing the markings that were placed on the golfballs fabricated in the examples and the comparative examples.

DETAILED DESCRIPTION OF THE INVENTION

The objects, features and advantages of the invention will become moreapparent from the following detailed description, taken in conjunctionwith the foregoing diagrams.

The structure of the practice golf ball of the invention is exemplifiedby, as shown in FIG. 1, a two-piece solid golf ball G having a core 1and a cover 2 which encases the core 1. The cover 2 typically has aplurality of dimples D formed on a surface thereof. In the diagram, thecore 1 and the cover 2 are shown as single layers, although the core andthe cover may each be composed of a plurality of layers.

The core is obtained by vulcanizing a rubber composition composedprimarily of a rubber material. The rubber composition used to form thecore includes a base rubber, a co-crosslinking agent, a crosslinkinginitiator, a metal oxide and, optionally, an antioxidant. The baserubber used in this rubber composition is preferably polybutadiene. Inthe invention, as will be subsequently described, the corecross-sectional hardness changes in specific ways from the surface tothe center of the core, and it is necessary to adjust the corecross-sectional hardness distribution, also referred to herein as the“core hardness profile,” within certain desired ranges. To this end, informulating the core, it is essential to suitably adjust, for example,the amounts in which the various subsequently described compoundingingredients are included, the vulcanization temperature and thevulcanization time.

The polybutadiene used as the rubber component must have a cis-1,4 bondcontent of at least 60 wt %, preferably at least 80 wt %, morepreferably at least 90 wt %, and most preferably at least 95 wt %. Ifthe cis-1,4 bond content is too low, the rebound may decrease. Inaddition, the polybutadiene has a 1,2-vinyl bond content of preferably 2wt % or less, more preferably 1.7 wt % or less, and even more preferably1.5 wt % or less.

The polybutadiene has a Mooney viscosity (ML₁₊₄ (100° C.)) which ispreferably at least 30, more preferably at least 35, even morepreferably at least 40, and most preferably at least 45, but ispreferably not more than 100, more preferably not more than 80, evenmore preferably not more than 70, and most preferably not more than 60.

The term “Mooney viscosity” used herein refers to an industrialindicator of viscosity (JIS K6300) as measured with a Mooney viscometer,which is a type of rotary plastometer. This value is represented by theunit symbol ML₁₊₄ (100° C.), wherein “M” stands for Mooney viscosity,“L” stands for large rotor (L-type), and “1+4” stands for a pre-heatingtime of 1 minute and a rotor rotation time of 4 minutes. The “100° C.”indicates that measurement was carried out at a temperature of 100° C.

In order to obtain the rubber composition in a molded and vulcanizedform which has a good rebound, it is preferable for the polybutadiene tohave been synthesized using a rare-earth catalyst or a Group VIII metalcompound catalyst.

The rare-earth catalyst is not subject to any particular limitation,although preferred use can be made of a catalyst which employs alanthanum series rare-earth compound. Also, where necessary, anorganoaluminum compound, an alumoxane, a halogen-bearing compound and aLewis base may be used in combination with the lanthanum seriesrare-earth compound. Preferred use can be made of, as the various abovecompounds, those compounds mentioned in JP-A 11-35633, JP-A 11-164912and JP-A 2002-293996.

Of the above rare-earth catalysts, the use of a neodymium catalyst thatemploys a neodymium compound, which is a lanthanide series rare-earthcompound, is especially recommended. In such a case, a polybutadienerubber having a high cis-1,4 bond content and a low 1,2-vinyl bondcontent can be obtained at an excellent polymerization activity.

The polybutadiene has a polydispersity Mw/Mn (Mw being theweight-average molecular weight, and Mn being the number-averagemolecular weight) of preferably at least 2.0, more preferably at least2.2, even more preferably at least 2.4, and most preferably at least2.6. The upper limit is preferably 6.0 or less, more preferably 5.0 orless, and even more preferably 4.5 or less. If Mw/Mn is too low, theworkability may decrease. On the other hand, if Mw/Mn is too high, therebound may decrease.

When the above polybutadiene is used as the base rubber, the proportionof the overall rubber represented by the polybutadiene is preferably atleast 40 wt %, more preferably at least 60 wt %, even more preferably atleast 80 wt %, and most preferably at least 90 wt %. The abovepolybutadiene may represent fully 100 wt % of the base rubber, although98 wt % or less is preferred, and 95 wt % or less is more preferred.

Illustrative examples of cis-1,4-polybutadiene rubbers which may be usedinclude the high-cis products BR01, BR11, BR02, BR02L, BR02LL, BR730 andBR51, all of which are available from JSR Corporation.

Rubber components other than the above polybutadiene may also be used inthe base rubber, insofar as the objects of the invention are attainable.Illustrative examples of rubber components other than the abovepolybutadiene include polybutadienes other than the above polybutadiene,and other diene rubbers such as styrene-butadiene rubbers, naturalrubbers, isoprene rubbers and ethylene-propylene-diene rubbers.

Isoprene rubbers which may be used include those having a cis-1,4 bondcontent of at least 60 wt %, preferably at least 80 wt %, and morepreferably at least 90 wt %, and having a Mooney viscosity (ML₁₊₄ (100°C.)) of at least 70, preferably at least 75, and more preferably atleast 80, with an upper limit of 90 or less, and preferably 85 or less.For example, the product IR2200 available from JSR Corporation may beused.

Styrene-butadiene rubbers which may be used include solution-polymerizedstyrene-butadiene rubbers and emulsion-polymerized styrene-butadienerubbers. Compared with emulsion-polymerized styrene-butadiene rubbers,solution-polymerized styrene-butadiene rubbers do not contain organicacids and low-molecular-weight components that arise from themanufacturing process, and thus have a poor processability. In addition,when a solution-polymerized butadiene-styrene rubber is used in acore-forming rubber composition, the seasonal (winter versus summer)difference in ball rebound is larger. On the other hand, when anemulsion-polymerized styrene-butadiene rubber is used in thecore-forming rubber composition, compared with when asolution-polymerized styrene-butadiene rubber is used, hardening of thecore due to temperature changes on account of seasonal differences canbe effectively prevented. Therefore, by using these two types ofstyrene-butadiene rubber having different qualities in a specific ratio,it is sometimes possible to suppress a decline in resilience and achange in hardness during winter use while maintaining a goodprocessability. By way of illustration, use may be made of thesolution-polymerized products SBR-SL552, SBR-SL555 and SBR-SL563(available from JSR Corporation) as the solution-polymerizedstyrene-butadiene rubber, and use can be made of theemulsion-polymerized products SBR 1500, SBR 1502, SBR 1507 and SBR 0202(available from JSR Corporation) as the emulsion-polymerizedstyrene-butadiene rubber. Ordinary, commercially available,solution-polymerized styrene-butadiene rubber has a styrene bond contentof from 5 wt % to 50 wt %, and emulsion-polymerized styrene-butadienerubber has a styrene bond content of from 15 wt % to 50 wt %.

The proportion of the overall rubber represented by rubber componentsother than polybutadiene is preferably 0 wt % or more, more preferablyat least 2 wt %, and most preferably at least 5 wt %, but is preferablynot more than 60 wt %, more preferably not more than 40 wt %, even morepreferably not more than 20 wt %, and most preferably not more than 10wt %.

In the invention, methacrylic acid is an essential ingredient which isemployed as the co-crosslinking agent. Methacrylic acid is included inan amount, per 100 parts by weight of the base rubber, of preferably atleast 10 parts by weight, more preferably at least 11 parts by weight,even more preferably at least 12 parts by weight, and still morepreferably at least 13 parts by weight. The upper limit in the amount ofmethacrylic acid is preferably not more than 40 parts by weight, morepreferably not more than 35 parts by weight, even more preferably notmore than 30 parts by weight, and most preferably not more than 25 partsby weight. Including too much methacrylic acid may make the core toohard, giving the ball an unpleasant feel on impact. On the other hand,including too little methacrylic acid may make the core too soft, alsogiving the ball an unpleasant feel on impact.

It is preferable to use an organic peroxide as the crosslinkinginitiator in this invention. Examples of commercial products that may beadvantageously used include Percumyl D, Perhexa 3M and Perhexa C40, allavailable from NOF Corporation. These may be used singly or ascombinations of two or more thereof.

The crosslinking initiator is included in an amount, per 100 parts byweight of the base rubber, of preferably at least 0.3 part by weight,more preferably at least 0.4 part by weight, even more preferably atleast 0.5 part by weight, and still more preferably at least 0.6 part byweight. The upper limit in the amount of crosslinking initiator ispreferably not more than 0.88 part by weight, more preferably not morethan 0.86 part by weight, and even more preferably not more than 0.84part by weight. Including too much crosslinking initiator may make thecore too hard, giving the ball an unpleasant feel on impact and alsosubstantially lowering the durability to cracking. On the other hand,including too little crosslinking initiator may make the core too soft,giving the ball an unpleasant feel on impact and also substantiallylowering productivity.

It is essential to use zinc oxide as the metal oxide in this invention,although metal oxides other than zinc oxide may also be used insofar asthe objects of the invention are attainable. The metal oxide is includedin an amount, per 100 parts by weight of the base rubber, of preferablyat least 15 parts by weight, more preferably at least 17 parts byweight, even more preferably at least 19 parts by weight, and mostpreferably at least 21 parts by weight. The upper limit in the amount ofmetal oxide is preferably not more than 30 parts by weight, morepreferably not more than 28 parts by weight, even more preferably notmore than 26 parts by weight, and most preferably not more than 24 partsby weight. Including too much or too little may make it impossible toobtain a suitable weight and a suitable hardness and rebound.

In the practice of the invention, an antioxidant may also be included inthe rubber composition. For example, use may be made of the commercialproducts Nocrac NS-6, Nocrac NS-30 and Nocrac 200, all available fromOuchi Shinko Chemical Industry Co., Ltd. These may be used singly or ascombinations of two or more thereof.

The amount of antioxidant included per 100 parts by weight of the baserubber, although not subject to any particular limitation, is preferablyat least 0.1 part by weight, and more preferably at least 0.2 part byweight, but is preferably not more than 1.0 part by weight, morepreferably not more than 0.7 part by weight, and even more preferablynot more than 0.4 part by weight. Including too much or too littleantioxidant may make it impossible to achieve a suitable core hardnessgradient, as a result of which a good rebound, good durability and goodspin rate-lowering effect on full shots may not be achieved.

In the practice of the invention, from a resource recycling standpoint,a ground or abraded powder of vulcanized rubber may be included in asmall amount of 40 parts by weight or less per 100 parts by weight ofthe base rubber. In such a case, the ground or abraded powder may becompounded in an amount, per 100 parts by weight of the base rubber,which is more than 0 wt %, preferably at least 2 wt %, and mostpreferably at least 5 wt %, but is preferably not more than 40 wt %,more preferably not more than 35 wt %, even more preferably not morethan 30 wt %, and most preferably not more than 25 wt %. The ground orabraded powder of vulcanized rubber is a vulcanizate which containsrubber and unsaturated carboxylic acid or a metal salt thereof. It isdesirable for the ground or abraded powder of vulcanized rubber used tohave a particle size which is preferably at least 20 μm, more preferablyat least 25 μm, and most preferably at least 30 μm, but is preferablynot more than 1,000 μm, more preferably not more than 900 μm, and mostpreferably not more than 800 μm. Adding a crushed or abraded powder ofvulcanized rubber has such effects as improving the productivity of thevulcanizate and increasing the durability to cracking. However,including too much may markedly lower the workability of the rubbercomposition and the productivity.

The core may be produced by using a known method to vulcanize and curethe rubber composition containing the various above ingredients. Forexample, production may be carried out by using a mixing apparatus suchas a Banbury mixer or a roll mill to mix the rubber composition,compression molding or injection molding the mixed composition in a coremold, then curing the molded body by suitable heating at a temperaturesufficient for the organic peroxide and co-crosslinking agent to act,such as under conditions of between about 100° C. and about 200° C. fora period of from about 10 minutes to about 40 minutes. The core hardnessprofile of the invention may be achieved by a combination of thevulcanization conditions and adjustment of the rubber formulation.

The core diameter, although not subject to any particular limitation, ispreferably at least 38.9 mm, and more preferably at least 39.3 mm, butis preferably not more than 42.1 mm, and more preferably not more than41.1 mm. At a core diameter outside of this range, the durability of theball to cracking may worsen dramatically and the initial velocity of theball may decrease.

The core must have a specific gravity of at least 1.05, preferably atleast 1.08, and more preferably at least 1.1, but not more than 1.2,preferably not more than 1.15, and more preferably not more than 1.13.

The core deflection under loading (referred to here and below as “CH”),i.e., the deflection by the core when compressed under a final load of1,275 N (130 kgf) from an initial load of 98 N (10 kgf), is at least 2.8mm, preferably at least 2.9 mm, and more preferably at least 2.95 mm,but is not more than 7.0 mm, preferably not more than 6.0 mm, morepreferably not more than 5.0 mm, and most preferably not more than 4.5mm. If the core deflection CH is too small, the feel of the practicegolf ball on impact will be so hard as to make the ball unpleasant touse. On the other hand, if the core deflection is too large, the feel ofthe practice golf ball on impact will be so soft as to make the ballunpleasant to use, in addition to which the productivity may declineconsiderably.

The core rebound (also referred to as the core initial velocity, CV), ispreferably at least 65 m/s, more preferably at least 68 m/s, even morepreferably at least 71 m/s, and most preferably at least 73 m/s, but ispreferably not more than 76 m/s, more preferably not more than 75.7 m/s,even more preferably not more than 75.4 m/s, and most preferably notmore than 75 m/s. A core rebound outside of this range is undesirablebecause the distance of the ball may dramatically decline or the ballmay fly so far as to pass over the netting at a driving range.

In the present invention, as shown in the schematic diagram of the corein FIG. 2, letting A be the JIS-C hardness at a surface of the core, Bbe the JIS-C hardness at a position 2 mm inside the core surface, C bethe JIS-C hardness at a position 5 mm inside the core surface, D be theJIS-C hardness at a position 10 mm inside the core surface, E be theJIS-C hardness at a position 15 mm inside the core surface, and F be theJIS-C hardness at the center of the core, it is essential for therespective values A to F to fall within the specific ranges indicatedbelow. By thus setting the hardness profile at the core interior withinspecific ranges, both a comfortable feel on impact similar to that of agame ball and also a good durability to cracking can be obtained.

Letting A be the JIS-C hardness at the surface of the core, the value ofA is at least 65, preferably at least 66, more preferably at least 67,and even more preferably at least 68, but is not more than 83,preferably not more than 80, more preferably not more than 77, and evenmore preferably not more than 74.

Letting B be the JIS-C hardness at a position 2 mm inside the coresurface, the value of B is at least 59, preferably at least 60, morepreferably at least 61, and even more preferably at least 62, but is notmore than 78, preferably not more than 74, more preferably not more than70, and even more preferably not more than 66.

Letting C be the JIS-C hardness at a position 5 mm inside the coresurface, the value of C is at least 61, preferably at least 62, morepreferably at least 63, and even more preferably at least 64, but is notmore than 80, preferably not more than 77, more preferably not more than74, and even more preferably not more than 71.

Letting D be the JIS-C hardness at a position 10 mm inside the coresurface, the value of D is at least 59, preferably at least 60, morepreferably at least 61, and even more preferably at least 62, but is notmore than 75, preferably not more than 73, more preferably not more than72, and even more preferably not more than 70.

Letting E be the JIS-C hardness at a position 15 mm inside the coresurface, the value of E is at least 56, preferably at least 57, morepreferably at least 59, and even more preferably at least 61, but is notmore than 70, preferably not more than 69.5, more preferably not morethan 69, and even more preferably not more than 68.5.

Letting F be the JIS-C hardness at the center of the core, the value ofF is at least 53, preferably at least 55, more preferably at least 57,and even more preferably at least 59, but is not more than 67,preferably not more than 66.5, more preferably not more than 66, andeven more preferably not more than 65.5.

Moreover, in the above core hardness profile, it is critical for therelative hardness conditions A>B<C≧D>E>F to be satisfied, for the valueA-F to be not more than 19, for the core to be formed in such a way thatA has the highest value among A to F, and for the value A-C to be in arange of from 0 to 8. If the above conditions are not satisfied, theball will have a diminished feel on impact and a reduced durability tocracking.

The value of A-C must be within a range of from 0 to 8, with the lowerlimit being preferably more than 0, and more preferably at least 1, andthe upper limit being preferably not more than 6, and more preferablynot more than 4. The value of A-F is not more than 19, with the lowerlimit being preferably at least 2, more preferably at least 4, and evenmore preferably at least 6, and the upper limit being preferably notmore than 16, more preferably not more than 13, and even more preferablynot more than 10.

In the practice of the invention, the core may be administered surfacetreatment with a solution containing a haloisocyanuric acid and/or ametal salt thereof.

Prior to surface-treating the core with a solution containing ahaloisocyanuric acid and/or a metal salt thereof, adhesion between thecore surface and the adjoining cover material can be further enhanced byabrading the surface of the core (referred to below as “surfacegrinding”).

Such surface grinding removes the skin layer from the surface of thevulcanized core, and thus makes it possible to enhance the ability ofthe solution of haloisocyanuric acid and/or a metal salt thereof topenetrate the core surface and also to increase the surface area ofcontact with the adjoining cover material. Exemplary surface grindingmethods include buffing, barrel grinding and centerless grinding.

The above haloisocyanuric acid and metal salt thereof is the compoundshown in the following formula (I).

In the formula, X is a hydrogen atom, a halogen atom or an alkali metalatom. At least one occurrence of X is a halogen atom. Preferred halogenatoms include fluorine, chlorine and bromine, with chlorine beingespecially preferred. Preferred alkali metal atoms include lithium,sodium and potassium.

Illustrative examples of the haloisocyanuric acid and/or a metal saltthereof include chloroisocyanuric acid, sodium chloroisocyanurate,potassium chloroisocyanurate, dichloroisocyanuric acid, sodiumdichloroisocyanurate, sodium dichloroisocyanurate dihydrate, potassiumdichloroisocyanurate, trichloroisocyanuric acid, tribromoisocyanuricacid, dibromoisocyanuric acid, bromoisocyanuric acid, sodium and othersalts of dibromisocyanuric acid, as well as hydrates thereof, anddifluoroisocyanuric acid. Of these, chloroisocyanuric acid, sodiumchloroisocyanurate, potassium chloroisocyanurate, dichloroisocyanuricacid, sodium dichloroisocyanurate, potassium dichloroisocyanurate andtrichloroisocyanuric acid are preferred because they are readilyhydrolyzed by water to form acid and chlorine, and thus play the role ofinitiating addition reactions to the double bonds in the diene rubbermolecules. The use of trichloroisocyanuric acid provides an especiallyoutstanding adhesion-improving effect.

The haloisocyanuric acid and/or a metal salt thereof is preferablydissolved in an organic solvent and used as a solution. A known organicsolvent may be used for this purpose, with the use of an organic solventwhich is soluble in water being especially preferred. Examples includeethyl acetate, acetone and methyl ethyl ketone. Of these, acetone isespecially preferred on account of its ability to penetrate the coresurface. The use of a water-soluble solvent is preferable because suchsolvents readily take up moisture; either the moisture which has beentaken up readily undergoes a hydrolysis reaction with thehaloisocyanuric acid and/or a salt thereof deposited on the core surfaceor, when water washing is used in a subsequent step, as the affinity tothe core surface increases, a hydrolysis reaction between the water andthe haloisocyanuric acid and/or a metal salt thereof more readilyarises.

When dissolved in an organic solvent, the content of the haloisocyanuricacid and/or a metal salt thereof in the solution is preferably at least0.3 wt %, more preferably at least 1 wt %, and more preferably at least2.5 wt %. At less than 0.3 wt %, the adhesion improving effectanticipated following core surface treatment may not be obtained,possibly resulting in a poor durability to impact. The upper limit inthe content may be as high as the saturated solution concentration.However, from the standpoint of cost effectiveness, when prepared as anacetone solution, for example, setting the upper limit in content to 10wt % is preferred. The core is immersed in the solution for a length oftime which is preferably at least 0.3 second, more preferably at least 3seconds, and even more preferably at least 10 seconds, but is preferablynot more than 5 minutes, more preferably not more than 1 minute, andeven more preferably not more than 30 seconds. If the immersion time istoo short, the desired effects of treatment may not be obtained, whereasif the immersion time is too long, a loss in ball productivity mayoccur.

The method of treating the core surface with a haloisocyanuric acidand/or a metal salt thereof is exemplified by methods which involvecoating the surface of the core with a solution of haloisocyanuric acidand/or a metal salt thereof by brushing or spraying on the solution, andmethods in which the core is immersed in a solution of thehaloisocyanuric acid and/or a metal salt thereof. From the standpoint ofproductivity and high penetrability of the core surface by the solution,the use of an immersion method is especially preferred.

After the core has been surface treated with a solution containinghaloisocyanuric acid and/or a metal salt thereof, it is preferable towash the surface of the core with water. Water washing of the coresurface may be carried out by a method such as running water, spraying,or soaking in a washing tank. However, because the aim here is notmerely to wash, but also to initiate and promote the desired treatmentreactions, too vigorous a washing method will not be appropriate.Therefore, preferred use may be made of washing by soaking in a washingtank. In such a case, it is desirable to place the cores to be washedfrom about one to five times in a washing tank that has been filled withfresh water.

Treating the core surface with a haloisocyanuric acid and/or a metalsalt thereof greatly improves adhesion between the core surface and thecover. The reason for this is not well understood, but is thought to beas follows.

The haloisocyanuric acid and/or a metal salt thereof, together with thesolvent, penetrates to the interior of the diene rubber making up thecore and approaches the vicinity of the double bonds on the backbone.Water then enters the core surface, whereupon the haloisocyanuric acidand/or a metal salt thereof is hydrolyzed by the water, releasing thehalogen. The halogen attacks the double bonds on the diene rubberbackbone located nearby, as a result of which an addition reactionproceeds. In the course of this addition reaction, the liberatedisocyanuric acid is added, together with the halogen, to the dienerubber backbone while retaining the cyclic structure. The addedisocyanuric acid has three —NHCO— structures on the molecule.

Because —NHCO— structures are thereby conferred to the core surface thathas been treated with the haloisocyanuric acid and/or a metal saltthereof, adhesion with the cover material improves further. It is mostlikely because of this that the durability of the golf ball to impactimproves. Moreover, when a polyurethane elastomer or polyamide elastomerhaving the same —NHCO— structures on the polymer molecule is used as thecover material, the affinity increases even further, presumablyincreasing the durability to impact.

When the addition of isocyanuric acid and chlorine (as the halogen) tothe surface of diene rubber has occurred, changes in the bonding statesbefore and after addition appear in an infrared absorption spectrum asincreases in the C═O bond (stretching) absorption peak at 1725 to 1705cm⁻¹, the broad H—H bond (stretching) absorption peak at 3450 to 3300cm⁻¹, and the C—Cl bond absorption peak at 800 to 600 cm⁻¹. Hence, bymeasuring the IR absorption spectrum of a surface-treated core andconfirming increases in these absorption peaks, it is possible toqualitatively confirm that isocyanuric acid and chlorine addition to thediene rubber molecules at the core surface has indeed occurred.

Following surface treatment, when the material at the surface portion ofthe solid core is examined by differential scanning calorimetry (DSC),no exothermic or endothermic peaks are observed from room temperature to300° C. This means that the functional groups which have been introducedmaintain a stable state within this temperature range. In other words,during molding of the cover material, the functional groups which havebeen introduced do not undergo degradation or the like due to heat, andcontinue to be effective. Also, because melting in the manner of a hotmelt resin does not arise, deleterious effects on durability and qualityof appearance, such as resin bleed out to the parting line, do notoccur. In addition, the very fact that the material in the surfaceportion of the solid core following the surface treatment describedabove is stable may be regarded as evidence that the isocyanuric acidhaving a melting point above 300° C. has been added with its molecularstructure still intact.

Next, the material making up the cover which directly encases the coreis described.

In this invention, the resin component in the cover is composed primaryof polyurethane. Use may be made of a thermoplastic polyurethaneelastomer or a thermoset polyurethane resin, with the use of athermoplastic polyurethane elastomer being especially preferred.

The thermoplastic polyurethane elastomer preferably has a structurecomposed of soft segments formed from a polymeric polyol (polymericglycol) and hard segments formed from a chain extender and adiisocyanate. Here, the polymeric polyol serving as a starting materialmay be any which has hitherto been used in the art relating tothermoplastic polyurethane materials, and is not subject to anyparticular limitation. Exemplary polymeric polyols include polyesterpolyols and polyether polyols. Polyether polyols are more preferablethan polyester polyols because thermoplastic polyurethane materialshaving a high rebound resilience and excellent low-temperatureproperties can be synthesized. Illustrative examples of polyetherpolyols include polytetramethylene glycol and polypropylene glycol.Polytetramethylene glycol is especially preferred from the standpoint ofthe rebound resilience and the low-temperature properties. The polymericpolyol has an average molecular weight of preferably from 1,000 to5,000. To synthesize a thermoplastic polyurethane material having a highrebound resilience, an average molecular weight of from 2,000 to 4,000is especially preferred.

The chain extender employed is preferably one which has hitherto beenused in the art relating to thermoplastic polyurethane materials.Illustrative examples include, but are not limited to, 1,4-butyleneglycol, 1,2-ethylene glycol, 1,3-butanediol, 1,6-hexanediol and2,2-dimethyl-1,3-propanediol. These chain extenders have an averagemolecular weight of preferably from 20 to 15,000.

The diisocyanate employed is preferably one which has hitherto been usedin the art relating to thermoplastic polyurethane materials.Illustrative examples include, but are not limited to, aromaticdiisocyanates such as 4,4′-diphenylmethane diisocyanate, 2,4-toluenediisocyanate and 2,6-toluene diisocyanate, and aliphatic diisocyanatessuch as hexamethylene diisocyanate. Depending on the type of isocyanate,control of the crosslinking reaction during injection molding may bedifficult. In this invention, the use of 4,4′-diphenylmethanediisocyanate, which is an aromatic diisocyanate, is most preferred.

A commercial product may be advantageously used as the thermoplasticpolyurethane material composed of the above materials. Illustrativeexamples include those available under the trade names Pandex T8180,Pandex T8195, Pandex T8290, Pandex T8295 and Pandex T8260 (all availablefrom DIC Bayer Polymer, Ltd.), and those available under the trade namesResamine 2593 and Resamine 2597 (available from Dainichiseika Color &Chemicals Mfg. Co., Ltd.).

It is essential for the cover to have a thickness which is at least 0.3mm, preferably at least 0.5 mm, and more preferably at least 0.7 mm, butis not more than 1.9 mm, preferably not more than 1.8 mm, and morepreferably not more than 1.7 mm. If the cover thickness is larger thanthe above range, the ball rebound will decrease, worsening the flightperformance. On the other hand, if the cover thickness is smaller thanthe above range, the durability to cracking will decrease. Inparticular, when the ball is hit thin, or “topped,” the cover is likelyto tear.

The cover has a specific gravity which is preferably at least 1.13, morepreferably at least 1.14, and even more preferably at least 1.15, but ispreferably not more than 1.30, more preferably not more than 1.20, andeven more preferably not more than 1.17.

It is critical for the cover material to have a Shore D hardness whichis at least 30, preferably at least 35, and more preferably at least 38,but is not more than 48, preferably not more than 47, more preferablynot more than 46, and even more preferably not more than 45. If theShore D hardness of the cover is higher than the above range, theappearance performance in long-term use (durability of markings) willdecline, in addition to which the flight performance will markedlydecrease. On the other hand, if the Shore D hardness of the cover islower than the above range, the durability to cracking will greatlydecrease and, particularly when the ball is topped, the cover is likelyto tear. In addition, the spin rate becomes very high, shortening thedistance traveled by the ball.

The practice golf ball of the invention has numerous dimples formed onthe surface thereof, each dimple having a spatial volume below a flatplane circumscribed by an edge of the dimple. It is critical for the sumof the dimple spatial volumes, expressed as a percentage (VR) of thevolume of a hypothetical sphere representing the ball were the ball tohave no dimples on the surface thereof, to be in a range of from 0.95%to 1.7%, the lower limit being preferably 0.97%, and more preferably0.98%, and the upper limit being preferably 1.5%, more preferably 1.3%,and most preferably 1.2%.

Also, although not subject to any particular limitation, the dimplesformed on the practice golf ball of the invention preferably satisfyconditions (1) and (2) below. Although it is preferable for both of thefollowing conditions (1) and (2) to be satisfied at the same time, it isacceptable for either one of these conditions alone to be satisfied.

First, referring to FIG. 3, as condition (1), it is preferable for thedimples to have a peripheral edge provided with a roundness representedby a radius of curvature R in a range of from 0.5 mm to 2.5 mm. Thelower limit of the radius of curvature R is more preferably 0.6 mm, andeven more preferably 0.7 mm, and the upper limit is more preferably 1.8mm, and even more preferably 1.5 mm.

Next, as condition (2), it is preferable for the ratio ER of acollective number of dimples RA having a radius of curvature R todiameter D ratio (R/D) of at least 20%, divided by a total number ofdimples N on the surface of the ball, to be in a range of from 15% to95%. Here, the ratio R/D is expressed as a percentage (R/D×100%), alarger value indicating a dimple in which the rounded part of the dimpleaccounts for a larger proportion of the dimple size and which has asmoother cross-sectional shape. The ratio ER indicates the number ofsuch smooth dimples as a proportion of the total number of dimples; bysetting ER in a range of from 15% to 95%, damage to the paint film atdimple edges can be effectively suppressed. The upper limit in the ratioR/D, although not subject to any particular limitation, is preferablynot more than 60%, and more preferably not more than 40%. The lowerlimit in the ratio ER is more preferably 20%, and even more preferably25%, and the upper limit is more preferably 90%, even more preferably85%, and most preferably 70%.

Also, although not subject to any particular limitation, it ispreferable for condition (3) to be satisfied. That is, as condition (3),it is preferable for the ball to have thereon a plurality of dimpletypes of differing diameter, and for the ratio DER of a combined numberof dimples DE obtained by adding together dimples having an own diameterand an own radius of curvature larger than or equal to a radius ofcurvature of dimples of larger diameter than the own diameter plusdimples of a type having a largest diameter, divided by the total numberof dimples N on the surface of the ball, to be at least 80%.

Generally, at a fixed dimple depth (see FIG. 3), the radius of curvatureR representing the roundness provided to the peripheral edges of thedimples is smaller at smaller dimple diameters D. However, abovecondition (3), by such means as adjusting the depth, sets the radius ofcurvature R representing the roundness of the peripheral edge to be aslarge as possible even in dimples having a small diameter D, thusforming dimples having a smooth cross-sectional shape and, by settingthe above ratio DER to at least 80%, increases the proportion of suchsmooth dimples, more effectively suppressing damage to the paint film.The ratio DER is more preferably at least 85%, even more preferably atleast 90%, and most preferably at least 93%. The upper limit in theratio DER is 100%.

In addition, the dimples formed on the practice golf ball of theinvention, although not subject to any particular limitation, preferablysatisfy conditions (4) to (6) below. Although it is preferable for allof the following conditions (4) to (6) to be satisfied at the same time,it is acceptable for any one of these conditions alone to be satisfied.

First, as condition (4), it is preferable for the number of dimple typesof differing diameter D on the ball to be 3 or more, and more preferablefor dimples of at least five types to be formed. In this case, thediameters D of the dimples, although not subject to any particularlimitation, are preferably set in a range of from 1.5 mm to 7 mm, thelower limit being more preferably 1.8 mm and the upper limit being morepreferably 6.5 mm. The depths of the dimples, although likewise notsubject to any particular limitation, are preferably set in a range offrom 0.05 mm to 0.35 mm, the lower limit being more preferably at least0.1 mm, and even more preferably at least 0.13 mm, and the upper limitbeing more preferably not more than 0.3 mm, and even more preferably notmore than 0.25 mm.

As condition (5), the total number of dimples N on the surface of theball is preferably not more than 380, and more preferably not more than350. The total number of dimples N is even more preferably in a range offrom 220 to 340.

As condition (6), it is preferable for the dimples to be formed in sucha way that the surface coverage SR of the dimples, which is the sum ofindividual dimple surface areas, each defined by a flat planecircumscribed by an edge of the dimple (dash-dot line in FIG. 3),expressed as a percentage of the surface area of a hypothetical sphererepresenting the ball were the ball to have no dimples on the surfacethereof (broken line in FIG. 3), is in a range of from 60% to 74%. At asurface coverage SR greater than 74%, the intervals between neighboringdimples become too narrow, which may make it difficult to provide thedimple edges with a roundness having the radius of curvature R specifiedin above condition (1). On the other hand, at a surface coverage SRbelow 60%, the aerodynamic performance decreases, as a result of whichthe distance traveled by the ball may decrease. The surface coverage SRhas a lower limit of more preferably 65%, and even more preferably 68%,and an upper limit of more preferably 73%.

In one-piece golf balls, because rubber has a somewhat yellow color, awhite enamel paint is generally applied as a first coat, following whicha clear paint is applied. In the inventive ball, in order to ensure agood appearance, it is preferable to apply a clear paint to the surfaceof the ball. The resulting clear coat has a thickness at dimple lands(Y) which is preferably at least 10 μm, more preferably at least 12 μm,and most preferably at least 13 μm, but is preferably not more than 30μm, more preferably not more than 25 μm, and most preferably not morethan 20 μm; and a thickness at dimple edges (Z) which is preferably atleast 8 μm, more preferably at least 10 μm, and most preferably at least11 μm, but is preferably not more than 28 μm, more preferably not morethan 23 μm, and most preferably not more than 18 μm. Also, the ratio Z/Yof edge areas (Z) to land areas (Y), expressed as a percentage, ispreferably at least 60%, more preferably at least 70%, and mostpreferably at least 80%, but is preferably not more than 100%, and morepreferably not more than 95%. Outside of the above range, the durabilityof markings at dimple edges decreases markedly in long-term use.

The ball diameter is preferably at least 42 mm, more preferably at least42.3 mm, and even more preferably at least 42.67 mm, but is preferablynot more than 44 mm, more preferably not more than 43.8 mm, even morepreferably not more than 43.5 mm, and most preferably not more than 43mm.

The ball weight is preferably at least 44.5 g, more preferably at least44.7 g, even more preferably at least 45.1 g, and most preferably atleast 45.2 g, but is preferably not more than 47.0 g, more preferablynot more than 46.5 g, and even more preferably not more than 46.0 g.

The ball has, upon initial measurement, a deflection BH1, whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf), of preferably at least 2.8 mm, more preferably atleast 2.85 mm, and even more preferably at least 2.9 mm, but preferablynot more than 7.0 mm, more preferably not more than 6.0 mm, even morepreferably not more than 5.0 mm, and most preferably not more than 4.5mm. When the core and the ball are each compressed under a final load of1,275 N (130 kgf) from an initial load of 98 N (10 kgf), lettingdeflection by the core be CH and deflection by the ball be BH1, theratio CH/BH1 is at least 0.95, preferably at least 0.96, and morepreferably at least 0.97, but is not more than 1.1, preferably not morethan 1.08, and more preferably not more than 1.07. If the ratio CH/BH1is too large, the deflection of the finished ball will be too smallrelative to the core deflection. That is, because the cover hardnessincreases, the feel on impact will decrease and the quality of theappearance will decline with long-term use. On the other hand, if theratio CH/BH1 is too small, the cover becomes very soft, whichsignificantly lowers the durability to cracking and leads in particularto cracking of the cover when the ball is topped. In addition, the spinrate becomes too high, resulting in a shorter distance of travel by theball.

In addition, the ball has, upon initial measurement, a rebound BV1 whichis preferably at least 65 m/s, more preferably at least 68 m/s, evenmore preferably at least 71 m/s, and most preferably at least 73 m/s,but is preferably not more than 76 m/s, more preferably not more than75.8 m/s, even more preferably not more than 75.6 m/s, and mostpreferably not more than 75.5 m/s. Outside of the foregoing range, thedistance traveled by the ball may greatly decrease, or the ball may flyso far as to pass over the netting at a driving range, making itunsuitable for use as a controlled-distance practice golf ball atdriving ranges.

Moreover, in order to achieve a good durability over a long period oftime, the practice golf ball of the invention, upon initial measurement,has a deflection BH1 (mm) when compressed under a final load of 1,275 N(130 kgf) from an initial load of 98 N (10 kgf) and an initial velocityBV1 (m/s) and, when measured again after being left to stand for 350days following initial measurement, has a deflection BH2 (mm) whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf) and an initial velocity BV2 (m/s), such that thedifference BH2−BH1 is preferably not more than 0.2 mm, more preferablynot more than 0.15 mm, and even more preferably not more than 0.1 mm,and such that the difference BV2−BV1 is preferably not more than 0.3m/s, more preferably not more than 0.2 m/s, and even more preferably notmore than 0.1 m/s.

EXAMPLES

The following Examples and Comparative Examples are provided toillustrate the invention, and are not intended to limit the scopethereof.

Examples 1 to 8 Comparative Examples 1 to 11

Rubber materials formulated as shown in Table 1 below were furnished forthe fabrication of practice golf balls in the Examples and ComparativeExamples. These rubber compositions were suitably mixed using a kneaderor roll mill, then vulcanized under the temperature and time conditionsin Table 1 to produce solid cores in the respective Examples. Ingredientamounts in the table below are shown in parts by weight.

TABLE 1 Type of Core (1) (2) (3) (4) (5) (6) (7) (8) Core BR01 100 60 95100 100 95 formulation IR2200 5 5 5 BR730 95 100 BR51 40 Perhexa C-400.6 0.6 (40% dilution) Actual amount 0.24 0.24 of addition Percumyl D0.8 0.8 0.8 0.8 1.2 0.6 0.6 0.8 Zinc oxide 23 23 23 22.5 23.5 9.5 9.5 23Antioxidant 0.2 0.2 0.2 0.2 0.2 0.1 0.1 0.2 Methacrylic acid 22.5 1822.5 13.5 29 22.5 Zinc methacrylate 26 Zinc acrylate 26 Titanium oxide 4Vulcanization Temperature (° C.) 170 170 170 170 170 160 160 170conditions Time (minutes) 20 20 20 20 20 13 13 30

Details on the materials used in the core formulations in the abovetable are provided below.

-   (1) BR01: A butadiene rubber synthesized with a nickel catalyst,    available from JSR Corporation; Mooney viscosity ML, 46-   (2) IR2200: An isoprene rubber, available from JSR Corporation;    Mooney viscosity ML, 82-   (3) BR730: A butadiene rubber synthesized with a neodymium catalyst,    available from JSR Corporation; Mooney viscosity ML, 55-   (4) BR51: A butadiene rubber synthesized with a neodymium catalyst,    available from JSR Corporation; Mooney viscosity ML, 36-   (5) Perhexa C-40: An organic peroxide, available from NOF    Corporation-   (6) Percumyl D: An organic peroxide, available from NOF Corporation-   (7) Zinc oxide: Available from Sakai Chemical Co., Ltd.-   (8) Antioxidant: “Nocrac NS-6,” available from Ouchi Shinko Chemical    Industry Co., Ltd.-   (9) Methacrylic acid: Available from Kuraray Co., Ltd.-   (10) Zinc methacrylate: Available from Asada Chemical Industry Co.,    Ltd.-   (11) Zinc acrylate: Available from Nihon Jyoryu Kogyo Co., Ltd.-   (12) Titanium oxide: Available from Ishihara Sangyo Kaisha, Ltd.

In each example, after the rubber composition formulated from theingredients shown in Table 1 was molded and vulcanized to form a core,the surface of the core was abraded to a desired diameter. Next, surfacetreatment of the core was carried out by immersing the core for 30seconds in an acetone solution of trichloroisocyanuric acid(concentration, 3 wt %), then washing the surface of the core withwater. The core was then set in a mold for injection molding the cover,and the cover composition shown in Table 2 below was injection moldedover the solid core. Ingredient amounts in the table below are shown inparts by weight.

TABLE 2 A B C D E F Resin Himilan 7331 50 Himilan 1557 50 30 Himilan1855 20 Himilan 1601 50 Pandex T8260 100 Pandex T8195 100 25 PandexT8290 75 Pandex T8180 100 Additives Magnesium 1 1 stearate Titanium 3.53.5 2.1 3.5 3.5 2.1 dioxide Polyethylene 1.5 1.5 1.5 1.5 wax Shore Dhardness 45 40 60 58 25 50

Details on the materials used in the cover composition in the abovetable are provided below.

-   “Himilan”: Ionomer resins available under this trade name from    DuPont-Mitsui Polychemicals Co., Ltd.-   “Pandex”: Thermoplastic polyurethane elastomers available under this    trade name from Dainippon Ink & Chemicals, Inc.-   Magnesium stearate: Available from NOF Corporation-   Titanium dioxide: Available under the trade name “Tipaque R550” from    Ishihara Sangyo Kaisha, Ltd.-   Polyethylene wax: Available under the trade name “Sanwax 161P” from    Sanyo Chemical Industries, Ltd.

In order to form a predetermined dimple pattern on the surface of thecover, a plurality of protrusions corresponding to the dimple patternwere formed in the mold cavity, by means of which dimples were impressedonto the surface of the cover at the same time that the cover wasinjection molded. Details on the dimples are given below in Table 3. Themarkings shown in FIG. 5 were printed on the ball surface.

In addition, the ball was clear-coated with a paint composed of 100parts by weight of polyester resin (acid value, 6; hydroxyl value, 168)(solids)/butyl acetate/PMA (propylene glycol monomethyl ether acetate)in a weight ratio of 70/15/15 as the base; 150 parts by weight of anon-yellowing polyisocyanate, specifically a hexamethylene diisocyanateadduct (available from Takeda Pharmaceutical Co., Ltd. as TakenateD-160N; NCO content, 8.5 wt %; solids content, 50 wt %) as the curingagent; and 150 parts by weight of butyl acetate. In Comparative Example11, a coating of white enamel paint was applied as a base coat for clearcoating.

TABLE 3 Dimple Diameter D Depth R R/D N RA ER DE DER SR VR No. Number(mm) (mm) (mm) ratio (number) (number) (%) (number) (%) (%) (%)Configuration Dimple I 1 24 4.4 0.207 0.7 16 338 102 30 330 98 72 0.99FIG. 4 2 204 4.2 0.200 0.7 17 3 66 3.6 0.190 0.8 22 4 12 2.7 0.160 0.8531 5 24 2.5 0.130 0.85 34 6 8 3.4 0.145 0.55 16 Dimple II 1 24 4.4 0.2160.5 11 338 36 11 306 91 72 0.99 FIG. 4 2 204 4.2 0.209 0.5 12 3 66 3.60.194 0.6 17 4 12 2.7 0.151 0.6 22 5 24 2.5 0.116 0.5 20 6 8 3.4 0.1600.5 15

The abbreviations and symbols relating to dimples which appear in Table3 are explained below.

-   R: Radius of curvature representing roundness provided at peripheral    edge of a dimple-   R/D ratio: Ratio of radius of curvature R to diameter D-   N: Total number of dimples on surface of ball-   RA: Collective number of dimples having an R/D ratio of at least 20%-   ER: Ratio of RA to total number of dimples N-   DE: Sum of number of dimples having an own diameter and an own    radius of curvature larger than or equal to a radius of curvature of    dimples of larger diameter than the own diameter, plus number of    dimples of a type having a largest diameter-   DER: Ratio of DE to total number of dimples N-   SR: Sum of individual dimple surface areas, each defined by a flat    plane circumscribed by an edge of the dimple, expressed as a    percentage of the surface area of a hypothetical sphere representing    the ball were the ball to have no dimples on the surface thereof.-   VR: Sum of individual dimple spatial volumes, each formed below a    flat plane circumscribed by an edge of the dimple, expressed as a    percentage of the volume of a hypothetical sphere representing the    ball were the ball to have no dimples on the surface thereof

The physical properties of the cores and covers in the respectiveexamples of the invention and the comparative examples, and the physicalproperties, distance, durability and feel of the practice balls obtainedin each example were measured or evaluated as described below. Theresults are presented in Tables 4 and 5.

Deflection of Core and Finished Ball (mm)

The deflection (mm) of the core or finished ball as the test sphere whencompressed at a rate of 10 mm/min under a final load of 1,275 N (130kgf) from an initial load state of 98 N (10 kgf) was measured. The testwas performed using a model 4204 test system from Instron Corporation.

Cross-Sectional Hardness of Core

The core was cut with a fine cutter and the JIS-C hardnesses at abovepositions B to F were measured in accordance with JIS K6301-1975 afterholding the core isothermally at 23±1° C. (at two places in each of N=5samples).

Surface Hardness of Core

JIS-C hardness measurements were carried out on the core surface inaccordance with JIS K6301-1975 after holding the core isothermally at23±1° C. (at two places in each of N=5 samples).

Rebound (Initial Velocity) of Core and Finished Ball

The initial velocity was measured using an initial velocity measuringapparatus of the same type as the USGA drum rotation-type initialvelocity instrument approved by the R&A. The cores or balls used as thesamples were held isothermally at a temperature of 23±1° C. for at least3 hours, then tested in a room temperature (23±2° C.) chamber. Ten ballswere each hit twice, and the time taken for the cores or balls totraverse a distance of 6.28 ft (1.91 m) was measured and used to computethe initial velocity.

Cover Material Hardness

A cover sheet was formed and, after holding the samples isothermally at23±1° C., the Shore D hardness was measured in accordance with ASTMD-2240.

Durability to Cracking

The ball was hit thin (“topped”) five times at the same place with theleading edge of a number nine iron (X-BLADE GR, manufactured byBridgestone Sports Co., Ltd.) at a head speed (HS) of 38 m/s, followingwhich the ball was repeatedly struck against a wall at an incidentvelocity of 43 m/s and the average number of shots (N=3 balls) untilcracking occurred was determined.

Abrasion Test

Ten golf balls and 3 liters of bunker sand were placed in a magneticball mill having an 8 liter capacity and mixing was carried out for 144hours, following which the balls were visually examined for any loss ofthe markings and to assess the degree of surface scratching, the degreeof loss of luster and the degree of sand adhesion. The ball appearancewas rated as “good,” “fair” or “NG.”

Measurement of Coating Thickness

-   Lands (Y): The thickness of the clear coat at land areas at    intermediate positions between dimples was measured.-   Edges (Z): The thickness of the clear coat at dimple edge areas was    measured.

The above measurements were carried out at three places on each of twoballs in the respective examples, and the average of these measurementswas determined.

Distance

A TourStage X-Drive 701 (loft angle, 9°), manufactured by BridgestoneSports Co., Ltd., was mounted as the driver (W#1) on a golf swing robotand struck at a head speed (HS) of 45 m/s. Both the spin rate of theball immediately after impact and the total distance traveled by theball were measured.

In addition, after the abrasion test described above had been carriedout, the total distance of the ball was again measured.

Feel

Ten teaching professionals hit the test balls with a driver (W#1) andrated the feel of the balls on impact as good, somewhat hard (fair), ortoo hard (NG).

TABLE 4 Example 1 2 3 4 5 6 7 8 Core Type (1) (2) (3) (4) (2) (2) (2)(2) Diameter, mm 39.9 39.9 39.9 39.9 39.3 40.5 41.1 39.9 Specificgravity 1.118 1.118 1.118 1.118 1.118 1.118 1.118 1.118 Deflection under10-130 kg 3 3.8 3.1 4.3 3.8 3.8 3.8 3.8 compression (CH), mm Rebound(CV), m/s 74.3 7 74.1 76 74.7 74.7 74.7 74.7 JIS-C hardness 73 69 72 6769 69 69 69 at core surface (A) JIS-C hardness 65 63 64 62 63 63 63 63 2mm inside core surface (B) JIS-C hardness 70 67 69 65 67 67 67 67 5 mminside core surface (C) JIS-C hardness 70 66 69 64 66 66 66 66 10 mminside core surface (D) JIS-C hardness 68 64 67 62 64 64 64 64 15 mminside core surface (E) JIS-C hardness 65 61 64 59 61 61 61 61 at corecenter (F) JIS-C hardness difference between core 3 2 3 2 2 2 2 2surface and 5 mm inside core (A − C) JIS-C hardness difference betweencore 8 8 8 8 8 8 8 8 surface and center (A − F) Cover Type A A A A A A AB Shore D hardness 45 45 45 45 45 45 45 40 Specific gravity 1.15 1.151.15 1.15 1.15 1.15 1.15 1.15 Thickness, mm 1.4 1.4 1.4 1.4 1.7 1.1 0.81.4 Finished Deflection under 10-130 kg 2.9 3.9 3 4.1 3.8 3.9 3.9 3.9ball compression 30 days after production (BH1), mm Deflection under10-130 kg 2.8 3.8 2.9 4 3.7 3.8 3.8 3.8 compression 350 days aftermeasuring BH1 (BH2), mm Difference between BH1 and BH2, mm −0.1 −0.1−0.1 −0.1 −0.1 −0.1 −0.1 −0.1 Rebound 30 days after production 73.8 74.173.5 75.4 73.9 74.3 74.5 74.3 (BV1), m/s Rebound 350 days aftermeasuring 73.8 74.2 73.6 75.3 74 74.4 74.5 74.3 BV1 (BV2), m/sDifference between BV1 and BV2, m/s 0 0.1 0.1 −0.1 0.1 0.1 0 0 Diameter,mm 42.7 42.7 42.7 42.7 42.7 42.7 42.7 42.7 Core initial velocity − 0.50.6 0.6 0.6 0.8 0.4 0.2 0.4 ball initial velocity (CV − BV1) Coredeflection/ball deflection (CH/BH1) 1.03 0.97 1.03 1.05 1.00 0.97 0.970.97 Dimples Type I I I I I I I I Clear Land areas (Y), μm 15 15 15 1515 15 15 15 coating Edge areas (Z), μm 13 13 13 13 13 13 13 13 thicknessCoating thickness ratio 88 88 88 88 88 88 88 88 (Z/Y × 100), % DistanceHS 45, driver Spin rate, rpm 3250 3090 3220 3050 2950 3180 3270 3170 (30days after Total distance, m 214 215 213 218 213 215 216 215 production)HS 45, driver Total distance, m 211 212 211 216 210 213 213 213 (afterabrasion test) Distance Total distance, m −3 −3 −2 −2 −3 −2 −3 −2difference Durability Durability to At incident 1103 1050 1095 1020 1335840 720 830 cracking velocity of 43 m/s Abrasion test After 144 goodgood good good good good good good (durability hours of sand ofmarkings) abrasion Feel Driver good good good good good good good good

TABLE 5 Comparative Example 1 2 3 4 5 6 Core Type (5) (2) (2) (6) (7)(7) Diameter, mm 39.9 38.5 42.3 39.9 39.9 39.9 Specific gravity 1.1281.118 1.118 1.118 1.118 1.118 Deflection under 10-130 kg 1.8 3.8 3.8 3.83.8 3.8 compression (CH), mm Rebound (CV), m/s 73.7 74.7 74.7 77 77.477.4 JIS-C hardness 90 69 69 70 70 70 at core surface (A) JIS-C hardness88 63 63 65 65 65 2 mm inside core surface (B) JIS-C hardness 87 67 6768 68 68 5 mm inside core surface (C) JIS-C hardness 81 66 66 66 66 6610 mm inside core surface (D) JIS-C hardness 74 64 64 62 62 62 15 mminside core surface (E) JIS-C hardness 70 61 61 58 58 58 at core center(F) JIS-C hardness difference between core 3 2 2 2 2 2 surface and 5 mminside core (A − C) JIS-C hardness difference between core 20 8 8 12 1212 surface and center (A − F) Cover Type A A A A A C Shore D hardness 4545 45 45 45 60 Specific gravity 1.15 1.15 1.15 1.15 1.15 0.99 Thickness,mm 1.4 2.1 0.2 1.4 1.4 1.4 Finished Deflection under 10-130 kg 1.9 3.63.9 3.8 3.8 3.3 ball compression 30 days after production (BH1), mmDeflection under 10-130 kg 1.9 3.5 3.9 3.5 3.5 3 compression 350 daysafter measuring BH1 (BH2), mm Difference between BH1 and BH2, mm 0 −0.10 −0.3 −0.3 −0.3 Rebound 30 days after production 74.4 73.6 74.7 76 76.477.0 (BV1), m/s Rebound 350 days after measuring 74.4 73.5 74.8 75.175.5 76.6 BV1 (BV2), m/s Difference between BV1 and BV2, m/s 0 −0.1 0.1−0.9 −0.9 −0.4 Diameter, mm 42.7 42.7 42.7 42.7 42.7 42.7 Core initialvelocity − −0.7 1.1 0.0 1.0 1.0 0.4 ball initial velocity (CV − BV1)Core deflection/ball deflection (CH/BH1) 0.95 1.06 0.97 1.00 1.00 1.15Dimples Type I II I I I I Clear Land areas (Y), μm 15 17 15 15 15 15coating Edge areas (Z), μm 13 8 13 13 13 13 thickness Coating thicknessratio 88 47 88 88 88 88 (Z/Y × 100), % Distance HS 45, driver Spin rate,rpm 3480 2860 3400 3100 3070 3040 (30 days after Total distance, m 223211 217 224 226 228 production) HS 45, driver Total distance, m 219 204214 221 223 213 (after abrasion test) Distance Total distance, m −4 −7−3 −3 −3 −15 difference Durability Durability to At incident 1425 1515585 555 518 300 cracking velocity of 43 m/s Abrasion test After 144 goodNG good good good NG (durability of markings) hours of sand abrasionFeel Driver NG good good good good good Comparative Example 7 8 9 10 11Core Type (7) (2) (2) (2) (8) Diameter, mm 39.9 39.9 39.9 39.9 42.7Specific gravity 1.118 1.118 1.118 1.118 1.121 Deflection under 10-130kg 3.8 3.8 3.8 3.8 compression (CH), mm Rebound (CV), m/s 77.4 74.7 74.774.7 JIS-C hardness 70 69 69 69 71 at core surface (A) JIS-C hardness 6563 63 63 65 2 mm inside core surface (B) JIS-C hardness 68 67 67 67 68 5mm inside core surface (C) JIS-C hardness 66 66 66 66 67 10 mm insidecore surface (D) JIS-C hardness 62 64 64 64 66 15 mm inside core surface(E) JIS-C hardness 58 61 61 61 63 at core center (F) JIS-C hardnessdifference between core 2 2 2 2 3 surface and 5 mm inside core (A − C)JIS-C hardness difference between core 12 8 8 8 8 surface and center (A− F) Cover Type A D E F Shore D hardness 45 58 25 50 Specific gravity1.15 1.15 1.15 0.99 Thickness, mm 1.4 1.4 1.4 1.4 Finished Deflectionunder 10-130 kg 3.8 3.40 3.9 3.6 3.1 ball compression 30 days afterproduction (BH1), mm Deflection under 10-130 kg 3.5 3.4 3.9 3.5 3.1compression 350 days after measuring BH1 (BH2), mm Difference betweenBH1 and BH2, mm −0.3 0.0 0 −0.1 0 Rebound 30 days after production 76.473.9 74.2 73.5 74.6 (BV1), m/s Rebound 350 days after measuring 75.574.0 74.2 73.6 74.7 BV1 (BV2), m/s Difference between BV1 and BV2, m/s−0.9 0.1 0 0.1 0.1 Diameter, mm 42.7 42.7 42.7 42.7 42.7 Core initialvelocity − 1.0 0.8 0.5 1.2 ball initial velocity (CV − BV1) Coredeflection/ball deflection (CH/BH1) 1.00 1.12 0.97 1.06 Dimples Type III I I I Clear Land areas (Y), μm 17 15 15 15 15 coating Edge areas (Z),μm 8 13 13 13 13 thickness Coating thickness ratio 47 88 88 88 88 (Z/Y ×100), % Distance HS 45, driver Spin rate, rpm 3070 2900 3520 3110 3650(30 days after Total distance, m 226 214 209 211 221 production) HS 45,driver Total distance, m 219 206 207 199 215 (after abrasion test)Distance Total distance, m −7 −8 −2 −12 −6 difference DurabilityDurability to At incident 518 1305 612 960 621 cracking velocity of 43m/s Abrasion test After 144 NG NG good NG NG (durability of markings)hours of sand abrasion Feel Driver good NG good good good

In the practice golf ball of Comparative Example 1, the deflection ofthe finished ball was too small. As a result, the ball had too hard afeel on impact, making it uncomfortable to use as a practice golf ball.

In the practice golf ball of Comparative Example 2, the cover was toothick, as a result of which the rebound was low, reducing the distance.In addition, the radius of curvature at the dimples edges was small andthe edge-to-land coating thickness ratio was small, as a result of whichthe durability of markings was very poor. Moreover, the flightperformance following the abrasion test (following evaluation of thedurability of markings) decreased.

In the practice golf ball of Comparative Example 3, the cover was toothin. As a result, the durability to cracking was poor, with thedurability to topping being particularly poor.

In the practice golf ball of Comparative Example 4, zinc methacrylatewas included as the co-crosslinking agent in the core-forming rubberformulation. As a result, the durability to cracking was poor.

In the practice golf ball of Comparative Example 5, zinc acrylate wasincluded as the co-crosslinking agent in the core-forming rubberformulation. As a result, the durability to cracking was very poor.

In the practice golf ball of Comparative Example 6, zinc acrylate wasincluded in the rubber composition and the cover was composed primarilyof an ionomer. As a result, the durability to cracking was very poor. Inaddition, because the cover was composed primarily of an ionomer, it wastoo hard, as a result of which the durability of markings was very poorand the flight performance following the abrasion test (followingevaluation of the durability of markings) greatly decreased.

In the practice golf ball of Comparative Example 7, zinc acrylate wasincluded in the core-forming rubber composition, as a result of whichthe durability to cracking was very poor. In addition, the radius ofcurvature at the dimple edges was small and the edge-to-land coatingthickness ratio was small, as a result of which the durability ofmarkings was very poor. Moreover, the flight performance following theabrasion test (following evaluation of the durability of markings)decreased.

In the practice golf ball of Comparative Example 8, the cover was toohard, as a result of which the durability of markings was very poor. Inaddition, the flight performance following the abrasion test (followingevaluation of the durability of markings) greatly decreased.

In the practice golf ball of Comparative Example 9, the cover was toosoft, as a result of which the durability to cracking was poor. When theball was topped in particular, the cover ended up cracking. In addition,the spin rate was too high, reducing the distance traveled by the ball.

In the practice golf ball of Comparative Example 10, the cover wasformed using an ionomer as the resin component. As a result, thedurability of markings was very poor. In addition, the flightperformance following the abrasion test (following evaluation of thedurability of markings) greatly decreased.

The practice golf ball of Comparative Example 11 was a one-piece golfball composed of a single layer. As a result, the durability to crackingwas poor and the spin rate was high. In addition, the decrease in theflight performance following the abrasion test (following evaluation ofthe durability of markings) was somewhat large.

Japanese Patent Application No. 2011-099945 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A practice golf ball comprising a core made of a rubber compositioncomprising a base rubber and, as compounding ingredients: aco-crosslinking agent, a crosslinking initiator and a metal oxide; and acover which encases the core and comprises a resin component, whereinthe co-crosslinking agent is methacrylic acid; the metal oxide is zincoxide; the core has a hardness profile in which, letting A be the JIS-Chardness at a surface of the core, B be the JIS-C hardness at a position2 mm inside the core surface, C be the JIS-C hardness at a position 5 mminside the core surface, D be the JIS-C hardness at a position 10 mminside the core surface, E be the JIS-C hardness at a position 15 mminside the core surface, and F be the JIS-C hardness at a center of thecore: A is from 65 to 83, B is from 59 to 78, C is from 61 to 80, D isfrom 59 to 75, E is from 56 to 70, and F is from 53 to 67; the relativehardness conditions A>B<C≧D>E>F are satisfied; the value A-F is not morethan 19; the core is formed in such a way that A has the highest valueamong A to F; the value A-C is from 0 to 8; the core has a specificgravity of from 1.05 to 1.2; the resin component of the cover iscomposed primarily of polyurethane; the cover has a thickness of from0.3 mm to 1.9 mm; the cover has a material hardness, expressed as theShore D hardness, of from 30 to 48; when the core and the ball are eachcompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf), letting deflection by the core be CH and deflection bythe ball be BH1, the core deflection CH is from 2.8 mm to 7.0 mm and theratio CH/BH1 is from 0.95 to 1.1; and the ball has formed on a surfacethereof a plurality of dimples, each dimple having a spatial volumebelow a flat plane circumscribed by an edge of the dimple, and the sumof the dimple spatial volumes, expressed as a percentage (VR) of thevolume of a hypothetical sphere representing the ball were the ball tohave no dimples on the surface thereof, being from 0.95% to 1.7%.
 2. Thepractice golf ball of claim 1, wherein the ball has, upon initialmeasurement, a deflection BH1 (mm) when compressed under a final load of1,275 N (130 kgf) from an initial load of 98 N (10 kgf) and an initialvelocity BV1 (m/s), and also has, when measured again after being leftto stand for 350 days following initial measurement, a deflection BH2(mm) when compressed under a final load of 1,275 N (130 kgf) from aninitial load of 98 N (10 kgf) and an initial velocity BV2 (m/s), suchthat the difference BH2−BH1 is not more than 0.2 mm and the differenceBV2−BV1 is not more than 0.3 m/s.
 3. The practice golf ball of claim 1,wherein the ball has formed on a surface thereof a plurality of dimpleswhich satisfy conditions (1) and (2) below: (1) the dimples have aperipheral edge provided with a roundness represented by a radius ofcurvature R of from 0.5 mm to 2.5 mm; and (2) the ratio ER of acollective number of dimples RA having a radius of curvature R todiameter D ratio (R/D) of at least 20%, divided by a total number ofdimples N on the surface of the ball, is from 15% to 95%.
 4. Thepractice golf ball of claim 3 which further satisfies condition (3)below: (3) the ball has thereon a plurality of dimple types of differingdiameter, and the ratio DER of a combined number of dimples DE obtainedby adding together dimples having an own diameter and an own radius ofcurvature larger than or equal to a radius of curvature of dimples oflarger diameter than said own diameter plus dimples of a type having alargest diameter, divided by the total number of dimples N on thesurface of the ball, is at least 80%.
 5. The practice golf ball of claim4 which further satisfies conditions (4) to (6) below: (4) the number ofdimple types of differing diameter is 3 or more; (5) the total number ofdimples N is not more than 380; and (6) the surface coverage SR of thedimples, which is the sum of individual dimple surface areas, eachdefined by a flat plane circumscribed by an edge of the dimple,expressed as a percentage of the surface area of a hypothetical sphererepresenting the ball were the ball to have no dimples on the surfacethereof, is from 60% to 74%.
 6. The practice golf ball of claim 1,wherein the polyurethane in the resin component of the cover is athermoplastic polyurethane elastomer.
 7. The practice golf ball of claim6, wherein the thermoplastic polyurethane elastomer comprises softsegments formed from a polymeric polyether polyol and hard segmentsformed from an aromatic diisocyanate.
 8. The practice golf ball of claim1, wherein the ball has, upon initial measurement, a deflection BH1 whencompressed under a final load of 1,275 N (130 kgf) from an initial loadof 98 N (10 kgf) of from 2.8 mm to 7.0 mm.
 9. The practice golf ball ofclaim 1, wherein the ball has, upon initial measurement, an initialvelocity BV1 of not more than 76 m/s.
 10. The practice golf ball ofclaim 1, wherein the compounding ingredients in the rubber compositionare included in respective amounts of from 10 to 40 parts by weight ofmethacrylic acid, from 15 to 30 parts by weight of metal oxide, from 0.3to 0.88 part by weight of crosslinking initiator, and from 0.1 to 1.0part by weight of an antioxidant, per 100 parts by weight of the baserubber.